motor and batteries for pilot boat question

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motor and batteries for pilot boat question

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22 May 2013 at 22:10 #41236
Michael Bord
Participant
@michaelbord10152
Last season I finished and ran my 28" scratch built pilot launch with a "generic" 545 motor and 6v lead acid 3.2 ah battery, with the help of some excellent advice from this forum.

Regreased prop shaft and rudder shaft this winter and yesterday was first trial outing this season (due to weather and lots of other things going on inc. daughter's wedding preparations!)

Anyway, yesterday was rather cold and windy so had lake to myself. Perhaps because of this I used more throttle than before and boat ran very well with (to me) realistic looking spray from bow, nice wash and about fast walking speed. I'd put a bit more ballast in stern and boat handled/ran better with the stern sitting lower and bow a little higher.

After 20 mins or so performance began to drop so I changed battery to my new NI-MH 4.2 ah pack. Boat ran a little faster at top speed but not much more.

After half hour or so including long sweeps up and down lake at full or 3/4 throttle, boat started slowing. Brought it in, found motor was very hot (too hot to touch) and there was a lot of weed round prop and trailing behind. Thought it best to stop at that point. Went home cleaned up boat and left it alone, apart from recharging batteries.

Today i removed prop shaft, cleaned up all traces of weed, replaced it. Shaft turns very freely. Ran motor light with 6v battery (controlled by transmitter/ speed controller). Motor gets quite hot, not too hot to touch, but don't think it got this hot before while running light.

Questions.

1. This is a "generic" 545 motor, no name or details on it but the windings have a 0.9 ohm resistance which gives it a 13 amp stall current (from a table I copied somewhere). It pulled 7 amps at full throttle in the bath with meter attached. I'm going to try it again (try boat running on low throttle in bath perhaps).

If the motor is damaged, what type/make of similar sized motor could be recommended to replace it? 545 power seems about right for the boat, doesn't need any more power I think?

2. My new NI-MH battery pack was sold as a 7.2 volt 4.2 ah pack. However, when I recharged it my meter showed a voltage of 8.4 v. It has 6 aa sized cells wrapped together. Is this actually 7.2 v?

If it is 8.4 v does this matter with the motor?

Speed controller is Mtroniks viper marine 15 (15 amp). I have a 10 amp in-line fuse in the 6v battery wiring (but not in the NI-MH pack).

Hope I haven't rambled too much but advice on motor and NI-MH battery pack welcomed.

I have to say that it was thoughly enjoyable to see boat running.

Michael
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22 May 2013 at 22:10 #2260
Michael Bord
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@michaelbord10152
23 May 2013 at 08:07 #41237
ashley needham
Participant
@ashleyneedham69188
Michael. is a 545 a low powered 540? if so an MFA540/1 motor will do on 7.2v. Without knowing exactly what motor you have it is difficult to say whet voltage it will take, but i would not have thought that 8.4v would harm the motor UNLESS it was a puny 3v type.. as you have been running this previousiously with no issues then it should be ok

All batteries will show above their rated voltage once fully charged. I suspect that your 8.4v will soon drop to 7.2 when under load.

Overheating. Obviously shouldnt do with no load, and if everything turns freely then you have no choice but to change something, motor being the easy choice. Only other thing I can think of is something tightening up on its own when the motor is going, that is not apparent when the motor is being turned over by hand. Are your nuts tight?

Ashley
23 May 2013 at 10:24 #41242
Charles Oates
Participant
@charlesoates31738
Hi Michael, the 545s seem to cover a large variety of voltage ranges, I have one that draws 3 amps on 12 volts and will run for hours and barely get warm. The running time seems about right, 7 amps from a 3.2 amp battery should give about 20 mins, but it's not ideal for a long battery life with a lead acid, but fine with a NI MH.

I recommend that you follow Ashleys advice and check everything is loose, and doesn't tighten up when running, especially the nuts, I hope he means the lock nuts at each end!

I think you will end up changing the motor, to keep the same speed you need something with a similar curent draw but not running so near its limit. There are others on here that can offer sugestions better than I can.

Charles
23 May 2013 at 12:13 #41244
Malcolm Frary
Participant
@malcolmfrary95515
Gathering the weed did the motor no favours, and may have caused it some damage, either windings, brushes or magnets. Getting hot under no load is not a good sign. A 545 is a 5 pole 540, more suited to torque than speed. A rule of thumb for figuring motor loading is that the prop diameter should not exceed the motor can diameter. Another that was given to me that seems to work is that the motor should have more poles than the prop has blades.

Too large a prop will overload the motor, as will introducing drag into the propshaft. A tube full of grease will keep water at bay, it will also make an effective brake when cold and stiff. Depending on the type of weed, this can either just reduce the efficiency of the prop, or it can add a lot of extra loading. A smaller prop can often result in a faster boat at the cost of some accelleration but with the benefits of a cooler motor and longer run time.
23 May 2013 at 14:10 #41245
Dave Milbourn
Participant
@davemilbourn48782
What type/size prop are you using e.g. "plastic, 3-blade, 35mm diameter"

Dave M
23 May 2013 at 19:49 #41248
Michael Bord
Participant
@michaelbord10152
Evening all, thanks for all the replies!

1. Motor is 50mm x 40mm. I think I managed to count 5 poles through a cooling slot befroe I fitted it but it was sold as a 545 – doesn't have any markings. Resistance of windings is 0.9 ohms if that helps with its characteristics. I could fit another motor of the same size easily into the boat (I've got a clamp I made up to hold the coupling straight while I line it up).

The boat doesn't really need any more power as it now runs as fast as the hull will allow, and looks the part at current top speed. I suppose lower current drain more torque rather than outright power would be the way to go.

Boat has run approx. 5 hours in 4 sessions so far. Motor didn't overheat when run previously, think it probably was the weed as the prop shaft now turns freely again. Perhaps I overgreased it. The nuts/coupling grub screws etc. all tight but it was being run faster as a trial this time.

2. The prop is 35mm 3 blade black Graupner. I was following the spec in an old Model Boats mag of a similar sized boat that had a slightly bigger prop and a 545 motor. But it would be a good experiment to try a smaller one.

I'm going to try running motor again but there was still a smell of something burnt when I took boat out of garage. (It's now in bath, sorry testing tank).

Cheers

Michael
23 May 2013 at 20:40 #41250
Dave Milbourn
Participant
@davemilbourn48782
Prop should be fine. I would concurr with Ashley's recommendation i.e. MFA 540/1. **LINK**
Dave M
23 May 2013 at 23:42 #41251
Michael Bord
Participant
@michaelbord10152
Thanks for link to MFA web site, Dave. I tried running motor with boat in bath but performance down and motor started to get far too hot again so I'll try a new motor. Prop shaft turned easily. The 540/1 type recommended sounds good and at £5.95 worth a go.

I'll report back when I've tried one.

thanks again to all above,

Michael
18 June 2013 at 22:58 #41652
Michael Bord
Participant
@michaelbord10152
Thanks to all above who supplied very helpful advice.

The short answer is that I've got the boat up and running again and it's going very well indeed. Hurrah!

The long answer is as follows:

Update on my boat's progress.

1. I bought an MFA 540/1 motor as suggested, also an MFA 385 motor for my Revell 1:72 scale Corvette project which was also suggested on another thread. I was very impressed by the service from MFA/Como drills. I ordered online late one evening and received the motors, both boxed and well wrapped, the day after next, so they must have been sent as soon as they got the order.

2. The new and old motors had the same sized casings. Took the old motor out of the boat, it looked in a pretty ropey state. I installed the 540/1, lined it up with the coupling clamped solid. All fine, shaft turns very freely as does motor. Took my little clamp off.

3. When I had some more time and having oiled bearings, I tried running motor with firstly a 1.5 volt battery directly connected to motor wires then the 6v one the boat uses. Using an ammeter the motor drew 0.5 amp running free out of water on 1.5v and .8 amp on 6v. In the testing tank (aka bath) it pulled 1.9 amps. Now the old motor drew nearly 7 amps with boat in water so it seemed to me that it was going to be about 1/3 power but would run for much longer.

4. Having today off, headed for model boat lake. Whoopee! First of all used 6v 3.2 a/h lead acid battery. Everything checked, boat in water and set off. Very quiet, ballasting I'd tried on previous run seemd to work with boat sitting well in water. After my last experience with weed, I brought the boat back for checking regularly but apart from one occasion, didn't get any round prop.

5. After some minutes I tried a full speed run. On the previous outing it had run at a fast walking speed which looked about a scale speed. This time, perhaps a little slower, but I suspect the previous motor produced more power than the boat needed for max hull speed although the wash looked quite satisfying. Anyway, cruised up and down lake for 55 mins (battery had previously only lasted 25 mins or so). Taking note of advice on this forum not to run down lead acid batteries too far, I changed the battery for a 7.2 v 4.2 a/h 6 sub-c cell one. Motor was perhaps slightly warm at this stage but not much.

6. The top speed of the boat on 7.2 v was now a bit faster, definitely as fast as I could walk, say 4mph, looked good to me without being too fast for scale speed. For a displacement hull probably wouldn't go much faster anyway. Motor very controllable at slower speeds as well.

Regrettably Tescos shopping called (and car park ticket ran out) so it was time to go. But I'd had 1hr 10mins sailing with no problems, second battery capacity hardly touched and duration of run now increased dramatically.

Conclusions –

a) MFA 540/1 very good motor for this boat, runs silently, controllable and enough power on 7.2 v. It's rated at 6 – 15 v so working well within it's capacity for my purposes.

b) Using lower drain motor increased run times greatly, without losing performance (on 7.2v) for this size of boat. Fine with 35mm 3 bladed prop.

c) Didn't really need the higher power of the "generic" 545 motor I first used, but of course I didn't know that until it was tried out.

d) Now have good motor/battery spec for future models of similar size and type, and one that won't break the bank.

c) MFA very good supplier.

So thanks again for all the helpful advice about my first radio controlled boat. Won't be the last. Next time I'll try a longer session using the 7.2 v pack and just cruise around for as long as possible.

Cheers

M.
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